Heteromorph ammonite5/6/2023 ![]() ![]() Orthoconic heteromorphs likely had a vertical orientation, whereas ancylocone heteromorphs had a near-horizontal aperture pointing upwards. Heteromorphs could achieve near-neutral buoyancy regardless of conch shape or ontog-eny. Forms with a U-shaped body chamber (ancylocone) are regarded as suspension feeders, whereas orthoconic forms additionally might have consumed benthic prey. Stomach contents of heteromorphs comprise planktic crustaceans, gastropods, and crinoids, suggesting a zooplanktic diet. Differences in radular tooth morphology and size in heteromorphs suggest a microphagous diet. All Cretaceous heteromorphs share an aptychus-type lower jaw with a lamellar calcitic covering. Heteromorphs likely lacked arm suckers, hooks, tentacles, a hood, and an ink sac. Based on phylogenetic bracketing with nautiloids and coleoids, hetero-morphs like other ammonoids had 10 arms, a well-developed brain, lens eyes, a buccal mass with a radula and a smaller upper as well as a larger lower jaw, and ammonia in their soft tissue. Their anatomy, buoyancy, locomotion, predators, diet, palaeoecology, and extinction are discussed. Combining direct evidence from their fossil record, indirect insights from phylogenetic bracketing, and physical as well as virtual models, we reach an improved understanding of heteromorph ammonoid palaeobiology. Since Wiedmann's seminal paper in this journal, the palaeobiology of heteromorphs has advanced substantially. Such aberrant forms appeared convergently four times within this extinct group of cephalopods. Heteromorphs are ammonoids forming a conch with detached whorls (open coiling) or non-planispiral coiling. This anomiid–heteromorph ammonite commensal relationship might continue to persist in descendants during the course of evolution of these heteromorph ammonites. Such colonization by anomiids is also observed on Didymoceras awajiense, which is considered to be the closely related ancestral species of P. It is suggested that fully mature individuals of this ammonite species lived for a long period of time after having formed the retroversal hook because a few generations of anomiids have colonized a single body chamber. sigmoidale did not lie on the sea floor and did not drag their body chambers. Attachment to both flanks and ventral peripheries of the retroversal hooks may indicate that at least adult individuals of P. ![]() These modes of occurrence suggest that the encrustation by anomiids occurred not on post-mortem floating or sunken carcasses but on live conchs and that these organisms were rapidly buried by turbidity current deposits shortly after death. sigmoidale conchs, attached predominantly to body chambers. Anomiids are found on both sides and ventral peripheries of P. sigmoidale are usually intact, and some individuals are associated with jaw apparatuses near apertures. Projecting retroversal hooks and apertures of P. sigmoidale with anomiids are often concentrated at the top of or just above turbidite sandstones. Distribution įossils of Ancyloceras species are found in the Cretaceous Barremian Stage (117-113 million year old) marine strata of Europe, Colombia and Morocco.The heteromorph ammonite Pravitoceras sigmoidale from the Upper Cretaceous Seidan Formation (Izumi Group) in south-west Japan is frequently encrusted by sessile anomiid bivalves. Most ammonites are homomorph, as they maintain the same shape throughout the growth, while the ammonites in this genus have uncoiled shells ( heteromorph or different-shaped ammonites), that would have precluded fast swimming. They are known as heteromorph shaped, with a partly uncoiled shell and the aperture directed toward the coiled part. Ancyloceras vandenheckii Astier, 1851 Īncyloceras ammonites have a shell reaching a length of about 10 centimetres (3.9 in) and a width of about 7 centimetres (2.8 in).Ancyloceras matheronianum d'Orbigny, 1842.Temporal range: Lower Barremian to Lower AptianĪncyloceras is an extinct genus of heteromorph ammonites found throughout the world during the Lower Cretaceous, from the Lower Barremian epoch until the genus extinction during the Lower Aptian. ![]()
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